A variety of spray guns are known in the art. An internal mix gun is often used when solvent emissions are a problem, because internal mixing limits the amount of atomized material and catalyst exiting the gun. Internal mix guns generally have three feed lines, a resin line and a catalyst line which feed into a manifold, and an air line. The resin and catalyst are typically mixed in the manifold. After mixing, the resin and catalyst are expelled from the gun in confluence through a nozzle or similar orifice with pressurized air from the air line. The pressurized air supplies sufficient pressure so that the resin and catalyst are sheared and atomized as they are expelled from the gun. A major drawback of this type of gun is that during a spraying operation, catalyzed resin often backs up into and catalyzes within the air supply. Catalyzed resin in the air supply leads to costly and time-consuming down time while the spraying operation is shut down and the air supply is cleared of any obstructions. Standard check valves are rarely effective as they quickly become hardened shut with catalyzed resin or the internal workings of the check valve become frozen with catalyzed resin.
A second type of gun typically used is an external mix gun. In an external mix gun, the resin and catalyst are atomized and expelled separately and directed toward one another. The resin and catalyst combine in the air shortly before contacting the article being treated. A major drawback of the external mix gun is the incomplete mixing of resin and catalyst, which often leads to patches of incompletely catalyzed resin appearing on the finished article. Such portions of uncatalyzed resin can produce points of weakness or blisters on the surface of the finished article.
A more important problem with external mix guns is the exterior atomization of the catalyst. Because of the incomplete mixing of the catalyst with the resin, much of the atomized catalyst disperses into the atmosphere and, more particularly, in the immediate work environment where the application is taking place. Concern over the safety of workers breathing catalyst contaminated air has led to numerous restrictions on the use of external mix guns.